1RCU Concepts 2 3 4The basic idea behind RCU (read-copy update) is to split destructive 5operations into two parts, one that prevents anyone from seeing the data 6item being destroyed, and one that actually carries out the destruction. 7A "grace period" must elapse between the two parts, and this grace period 8must be long enough that any readers accessing the item being deleted have 9since dropped their references. For example, an RCU-protected deletion 10from a linked list would first remove the item from the list, wait for 11a grace period to elapse, then free the element. See the listRCU.txt 12file for more information on using RCU with linked lists. 13 14 15Frequently Asked Questions 16 17o Why would anyone want to use RCU? 18 19 The advantage of RCU's two-part approach is that RCU readers need 20 not acquire any locks, perform any atomic instructions, write to 21 shared memory, or (on CPUs other than Alpha) execute any memory 22 barriers. The fact that these operations are quite expensive 23 on modern CPUs is what gives RCU its performance advantages 24 in read-mostly situations. The fact that RCU readers need not 25 acquire locks can also greatly simplify deadlock-avoidance code. 26 27o How can the updater tell when a grace period has completed 28 if the RCU readers give no indication when they are done? 29 30 Just as with spinlocks, RCU readers are not permitted to 31 block, switch to user-mode execution, or enter the idle loop. 32 Therefore, as soon as a CPU is seen passing through any of these 33 three states, we know that that CPU has exited any previous RCU 34 read-side critical sections. So, if we remove an item from a 35 linked list, and then wait until all CPUs have switched context, 36 executed in user mode, or executed in the idle loop, we can 37 safely free up that item. 38 39 Preemptible variants of RCU (CONFIG_TREE_PREEMPT_RCU) get the 40 same effect, but require that the readers manipulate CPU-local 41 counters. These counters allow limited types of blocking 42 within RCU read-side critical sections. SRCU also uses 43 CPU-local counters, and permits general blocking within 44 RCU read-side critical sections. These two variants of 45 RCU detect grace periods by sampling these counters. 46 47o If I am running on a uniprocessor kernel, which can only do one 48 thing at a time, why should I wait for a grace period? 49 50 See the UP.txt file in this directory. 51 52o How can I see where RCU is currently used in the Linux kernel? 53 54 Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu", 55 "rcu_read_lock_bh", "rcu_read_unlock_bh", "call_rcu_bh", 56 "srcu_read_lock", "srcu_read_unlock", "synchronize_rcu", 57 "synchronize_net", "synchronize_srcu", and the other RCU 58 primitives. Or grab one of the cscope databases from: 59 60 http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html 61 62o What guidelines should I follow when writing code that uses RCU? 63 64 See the checklist.txt file in this directory. 65 66o Why the name "RCU"? 67 68 "RCU" stands for "read-copy update". The file listRCU.txt has 69 more information on where this name came from, search for 70 "read-copy update" to find it. 71 72o I hear that RCU is patented? What is with that? 73 74 Yes, it is. There are several known patents related to RCU, 75 search for the string "Patent" in RTFP.txt to find them. 76 Of these, one was allowed to lapse by the assignee, and the 77 others have been contributed to the Linux kernel under GPL. 78 There are now also LGPL implementations of user-level RCU 79 available (http://lttng.org/?q=node/18). 80 81o I hear that RCU needs work in order to support realtime kernels? 82 83 This work is largely completed. Realtime-friendly RCU can be 84 enabled via the CONFIG_TREE_PREEMPT_RCU kernel configuration 85 parameter. However, work is in progress for enabling priority 86 boosting of preempted RCU read-side critical sections. This is 87 needed if you have CPU-bound realtime threads. 88 89o Where can I find more information on RCU? 90 91 See the RTFP.txt file in this directory. 92 Or point your browser at http://www.rdrop.com/users/paulmck/RCU/. 93 94o What are all these files in this directory? 95 96 See 00-INDEX for the list. 97